In many typical state of the art X-ray tubes, a cathode assembly and an anode assembly are vacuum sealed in a glass or metal envelope. Electrons are generated by a cathode filament in the cathode assembly. These electrons are accelerated toward the anode assembly by a high voltage electrical field. The high energy electrons generate X-rays upon impact with the anode assembly. A by-product of this process is the generation of substantial amounts of heat.
Traditional X-ray tube configurations are known in the prior art, for example, Coolidge type X-ray tubes. In a Coolidge tube X-ray photons, shown as a spot output radiation pattern, are generated by impinging an electron beam emanating from filament onto the surface of a target anode. Coolidge tubes may be operated single ended with the cathode at a negative potential and the anode at ground, or double ended with the cathode at a negative potential and the anode at a positive potential. In either configuration the energy of acceleration is the difference between the electrode potentials. In a Coolidge X-ray tube the target anode is fabricated from a heavy metal such as tungsten, tantalum or iridium and such materials are selected because of their density and high melting point. The material of the target anode is most often mounted onto a thermally conductive material such as copper and is externally cooled either by water or dielectric oil.
The target anode is placed in line with the electron beam and radiation is emitted at right angles to the electron beam. The spectrum of the output radiation is predominantly bremsstrahlung and is altered by changing the accelerating energy of the electron beam. Tubes of this nature are in use in industrial imaging, medical imaging, analytical and irradiation application. The primary limitation of this type of tube is the watt density loading of the target anode before melting occurs, limited utilization of generated X-ray photons and the symmetry of the resulting radiation field. Because the resolution of an imaging device, either electronic or film, is a function of the size of the electron beam projected onto the target anode. For optimal image resolution a small focal spot is desired, but for optimal image contrast a large number of X-ray photons are desired. The two requirements are contrary and cannot be resolved in the traditional tube design. In addition the reflective nature of the emitted radiation is asymmetrical about a beam centerline and is grossly inefficient for X-ray irradiation applications.
Recently, some low power through transmission elongated X-ray tubes has become available on the market. These tubes utilize a single filament and the filament does not heat uniformly rather the filament tends to heat up in the center generating most electrons in the center of the filament, and thus causing a short or narrow linear Gaussian radiation output pattern.
Therefore, it is readily apparent that there is a recognizable unmet need for a longitudinal high dose output, through transmission target X-ray system and methods of use, having a large surface area anode target to dissipate heat, and thus, enabling multiple shortened cathode filaments, higher atomic number target material with improved radiation output, lower melting point and higher vaporization pressure, and lower electrode potential required to produce higher output radiation.